This invention relates to a fluidized bed steam generation system and a method of operating same and, more particularly, to such a system and method in which recycled flue gases are used to assist in passing separated solids from a separator section to a furnace section.
Fluidized bed steam generation systems are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed system offers an attractive combination of high heat release, high sulphur adsorption, low nitrogen oxide emissions, and high fuel flexibility.
The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface.
Other types of fluidized beds utilize a "circulating" fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, the air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
These circulating fluidized bed systems are characterized by relatively high solids recycling which makes the system insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling also improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
In the event the reactor is in the form of a steam generator, the walls of the reactor are usually formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, which separates the water from the converted steam, which is routed either to a turbine to generate electricity or to a steam user.
In these arrangements, the gaseous product from the furnace is often passed through a cyclone separator, which separates the entrained solid particulate material from the gaseous mixture and recycles the solid particulate material back into the furnace through a loopseal and a J-valve. The gaseous remainder from the cyclone separator is passed through a heat recovery section and to a baghouse in which the gases are drawn through bag filters using an induction draft fan to separate any remaining fine particulate material from the gases.
In transporting the solid particulate material separated in the cyclone separator back into the furnace, air has been used to assist the movement of the material through the loopseal and into the furnace. However, when the recycled particulate material contains fine-size char it often combusts if air, which typically contains approximately 21% oxygen, is used to assist in passing the solid particulate material through the loopseal. The combustion raises the temperature of the material in the loopseal to relatively high levels. Further, when low grade fuels are used that contain a moderate to high amount of vanadium in their ash, low eutectic vanadium oxide compounds are formed when mixed with air. The combination of increased temperature caused by the combustion and the agglomeration of vanadium result in plugging of the loopseal which can cause shutdown and considerably reduce the efficiency of the system.